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Parser.java
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Parser.java
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import java.io.InputStream;
import java.io.IOException;
class Parser{
private int lookaheadToken;
private InputStream in;
public Parser(InputStream in) throws IOException{
this.in = in;
lookaheadToken = in.read();
}
public int eval() throws IOException, ParseError{
int value = exp1();
if(lookaheadToken != '\n' && lookaheadToken != -1){ //if given expresion is not over
throw new ParseError();
}
return value;
}
//same as in parse example
private void consume(int symbol) throws IOException, ParseError{
if (lookaheadToken != symbol)
throw new ParseError();
lookaheadToken = in.read();
}
//exp1 -> term1 exp2
private int exp1() throws IOException, ParseError{
//the expresion must start with a number or "(" because first(term1) = {num, (}
if ((lookaheadToken >= '0' && lookaheadToken <= '9') || lookaheadToken == '('){
return exp2(term1());
}
else{
throw new ParseError();
}
}
//exp2 -> ^ term1 exp2 | ε
private int exp2(int value) throws IOException, ParseError{
if (lookaheadToken == '^'){
consume(lookaheadToken);
return exp2(value^term1()); //previus value given ^ value return from term1 function
}
else if((lookaheadToken >= '0' && lookaheadToken <= '9') || lookaheadToken == '('){
throw new ParseError();
}
return value;
}
//term1 -> factor term2
private int term1() throws IOException, ParseError{
if ((lookaheadToken >= '0' && lookaheadToken <= '9') || lookaheadToken == '('){
return term2(factor());
}
else{
throw new ParseError();
}
}
//term2 -> & factor term2 | ε
private int term2(int value) throws IOException, ParseError{
if (lookaheadToken == '&'){
consume(lookaheadToken);
return term2(value&factor()); //calculates value given &
}
else if((lookaheadToken >= '0' && lookaheadToken <= '9') || lookaheadToken == '('){
throw new ParseError();
}
return value;
}
//factor -> num | (exp1)
//num -> 0|1|..|9
private int factor() throws IOException, ParseError{
int value = -1;
if (lookaheadToken >= '0' && lookaheadToken <= '9'){
value = lookaheadToken - '0';
consume(lookaheadToken);
}
else if(lookaheadToken == '('){
consume(lookaheadToken);
value = exp1();
consume(')'); //if next to exp1 is not ")" consume fuction will throw parse error
}
else{
throw new ParseError();
}
return value;
}
}